Hanging Out

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The suspension rope supporting a temporary platform is the single most important element of a suspended scaffold. You may not agree with this—too bad for you. What if the rope breaks? The platform can only go down and if you are at a considerable height, the result will be mostly unpleasant. Understanding this suggests that we should probably be sensitive to the condition of the rope to which we trust our lives.

What is a rope? A typical definition describes a rope as a cord that consists of twisted strands of material, such as hemp or wire. Of course, that begs the question of what cords and strands are. For that matter what is hemp? Can you smoke it? Perhaps not. How about this: a rope is a bunch of string or thread twisted together to make a bundle that can hold some weight. In the case of suspended scaffolds, the strings are normally wire although other materials such as hemp and polypropylene can be used, depending on the application.

Rope has been around just about forever. Evidence of rope’s use shows up in ancient Asia and Egypt. Wire ropes were invented about 1831 or so by Wilhelm Albert, a German involved with mining. He sought a solution to the very real problem of using chains where the failure of one link meant the failure of the whole chain. By twisting individual wires/strings into small bundles (strands) and then twisting the strands into a rope, (a big bundle), any defects are spread over more components, thus avoiding the problem of the weak link.

The industrial revolution encouraged rapid development of wire rope technology and the use of wire rope continued to increase. In 1841, John A. Roebling, designer and constructor of the Brooklyn Bridge, began manufacturing wire rope in America. Continued research and development discovered that more wires in the rope offered more flexibility and in 1884, researcher Tom Seale developed the parallel strand, where he used different diameter strands to make the rope. Figure 1 illustrates the Seale design.

While iron wire was initially used for metal ropes, steel wire began to be used in the late 1800’s. In fact, steel wire rope was first used in the construction of the Brooklyn Bridge in New York; the main ropes are still in use, demonstrating the durability and longevity of wire ropes. Over time, other wire rope designs have appeared, including the Filler strand, the Warrington strand and the Lang lay rope. Each design has its advantages and the job requirements will dictate the choice.

Wire rope is strong stuff, especially considering its relative light weight. Wire rope load capacity is governed by the rope material, configuration and diameter. While wire rope is available in an almost infinite number of diameters, normal diameters for suspended scaffolds are 5/16 or 3/8 inches. By its nature, rope can only handle tensile loads (you can’t push a rope!). However, the great advantage of a rope is that it can still handle the rated load whether the rope is 5 feet or 500 feet long. Within limits, that means the rope can hang down a 300-foot tall building and still support the same load as the rope will on a 50-foot tall building.

Adjustable suspended scaffolds typically use drum hoists or traction hoists. Drum hoists wind the wire rope on a drum or spool attached to the scaffold platform while a traction hoist passes the rope through the machine. Consequently, a drum hoist and rigging must support the weight of the wire rope while a traction hoist does not.
As with all materials, wire rope, while rather durable, can be damaged by improper handling and use and can also just wear out through continued use. Consequently, suspended scaffold erectors, and users, must be adequately trained in the potential hazards. For example, erectors must know how to handle the wire rope, including how to pay out the rope and how to wind it back up at the end of the job. The rope must be installed so the bottom end of the rope can hang free.

The attachment of the rope to its anchor is obviously critical to the strength of the suspension system. At a minimum, when loops in a rope are being made, a thimble and three fist grips (no u-bolts please) must be used, spaced at the manufacturer’s recommendations. The bolts must be tightened in compliance with the manufacturer’s recommendations; they must be re-tightened after the first loading of the suspension system, and then typically every day after that. The entire scaffold system including the suspension rope, must be inspected prior to each workshift. Properly trained suspended scaffold operators will know to inspect the suspension wire ropes every time the platform is raised or lowered to ensure that the rope is still in useable condition. It is rather undesirable to get the rope stuck in the traction hoist when 100 feet in the air. Even less desirable is having the suspension rope break when 100 feet in the air!

Suspended scaffolds get some impressive media coverage when failures occur since the incident leaves workers dangling high above the street below. Reporters nervously describe the precarious (and assume a dangerous) situation, leading the uninformed observer to believe that these devices are incredibly unsafe and a peril to the users. Since wire ropes on properly designed scaffolds can support six times the expected load, when the scaffold fails, it isn’t because the equipment is hazardous, but rather it is because somebody just plain screwed up. Don’t you be one of them!

5 Essential Facts About Facade Access Design

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Facade design is an important aspect of any building project, but that doesn’t mean simply considering what the exterior of the finished building will look like.

Facade access design is an essential consideration. A good system will allow for maintenance to take place safely and at a reasonable cost. Maintenance operations can include setting up advertising, cleaning windows, fixing damage and more.

Here are five essential things you need to know about facade access design.

There are temporary solutions…

Temporary solutions for facade access include rope descent systems and hydraulic access platforms.

While these are relatively low-cost solutions, they have their drawbacks too. For example, a hydraulic lift will not reach the highest floors of a tall building, while rope descent work can take quite a long time.

… and permanent solutions

To secure a re-usable solution, a range of systems including monorail cradles or fixed davits might be favored depending on the jurisdiction (California, New York and other states have their own set of specific regulations).

Monorail cradles are useful on large flat or curved surfaces – they travel along a rail at the top of the building and can be lowered to the required level for access to the facade. They may not be appropriate to use for more ‘experimental’ facade designs.

For flat surfaces with less width, a fixed davit may be more cost-effective than a monorail cradle. Fixed davits are single arms which sit in one position and are used to raise and lower a maintenance platform.

Whichever of the two solutions you opt for, permanent or temporary, you will need to ensure that there is also a fall protection system to protect the people who are working on the facade.

Equipment needs to be inspected regularly

Just as facades need to be accessible, so does your facade access system so that it can be inspected and tested for safety at regular intervals.

OSHA 1910.66 states that all permanent equipment used to access a facade must be load tested when installed, and visually inspected every year. Additionally, OSHA 1910.27 states that each anchorages must be inspected annually and re-tested every 10 years.

Novel facade design calls for a novel approach

As modern architects create buildings with new and artistic facades, it’s important to think about how the facade will be accessible for maintenance purposes.

Sometimes, this will require an approach which is slightly different from the norm. This must be considered at an early stage of the project.

If the architect’s vision for facade design would result in a building which causes problems for facade access, there may have to be a compromise – or a novel approach.

It’s always good to get a second opinion

Our facade access design consultancy services allow building owners and architects to take advantage of our expert knowledge to create facade access designs that are safe and cost-effective.

We provide turnkey structural design and engineering solutions for new buildings, and can also help retrofit existing buildings to bring them up to code.  Contact Us today to find out how we can help with your façade access project.

I’m Digging Your Shoring Plan!

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Due to the complexity and property line constraints of modern construction, earth shoring requires a solution that must conform to both engineering and safety guidelines during all stages of construction.

Here are the 5 key concepts to remember for an earth shoring design:

1. Applicable codes: The type of project will define the requirements for an engineered earth shoring plan. For instance, a design that allows for inches of deflection at a multi-story urban high-rise may not be compatible with AREMA requirements for railroad earth shoring. While a contractor may be able to get away with using a cantilevered design, a similar design that incorporates the locomotive surcharge loads into the analysis may fail simply by being out of tolerance for railroad deflection guidelines. In this case, the common solution is to add soil anchors to keep the design in compliance.

2. Material: This is typically a contractor preference. If a contractor has a substantial inventory of steel I-beams/H-piles and wood lagging, it is in the best interest of the client for the engineer to design the system accordingly. Piles may need to be spaced more tightly and the design may not be as efficient as sheet piles, but it does eliminate the need for the contractor to spend more money.

3. Sequencing: With most earth shoring designs, there is a sequence of installation that must be followed based on the applied loads that change with depth. For example, in a cofferdam design, if wale frames are required, the contractor may have to install the wale at a specified elevation prior to proceeding. This elevation may be above the final excavation depth, but the engineer should have determined that this is the maximum depth that the shoring can support in a cantilevered condition and/or without restraint at the base. This may be a result of deep excavations where the substrate alone at the base is not adequate to support the lateral load. Oftentimes, many scenarios must be analyzed to ensure that the members are not overloaded and the entire shoring design is code-compliant at any given stage.

4. Embedment Depth: As a general guideline, the minimum embedment depth of a pile must be 75% of the retained height to ensure adequate development and base restraint.

5. Workers at the Top of the Excavation:  While the designer may account for the surcharge loads at the top of the excavation, it is also important to consider the impact of workers. If a guardrail is required based on project conditions, then it must be OSHA compliant and any loads/connections required shall be accounted for in the design of the pile. Common practice is to weld a guardrail post at the top of the pile, but this must be checked not only for load application, but also for maximum spacing.

Engineered shoring plans are critical components of construction plans, and a well-thought out design will save the contractor both time and money. As the saying goes,”Think before you dig!”

5 Impressive Things Built (or Fixed) Using Cofferdams

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Everyone knows about dams. But have you heard about a cofferdam?

Cofferdams have been around for a long time. People have used these when excavating very large plots of land or building foundations of water-based structures such as bridges or piers. The cofferdam keeps water from flowing into these sites, ensuring a dry foundation.

The cofferdam has been used to build and fix some impressive things. Check out the five most inspiring objects constructed by using these fascinating dams.

5 Impressive Things Built (or Fixed) Using Cofferdams

Cofferdams have helped civilizations divert water, gain new territory, build dry structures safely, and even recover history. They can be as simple as a pile of sandbags set up to use as a barrier during wartime or complex as a double sheet piling used in modern-day bridge construction.

While today the cofferdam is particularly useful for earth shoring engineering projects, it continues to be used in the engineering world as a helpful tool in water diversion projects.

1. Battleship U.S.S. North Carolina

Because ships are a water-borne craft, their preservation often depends upon a dry work environment. When it comes to this battleship located in Wilmington, North Carolina, the use of a cofferdam will integrate a memorial walkway for visitors and water-free access to the battleship for preservation and repair work.

The project, nearly six months away from completion, is unique because it won’t rely on the cofferdam for underwater construction. This battleship will be open to visitors and kept looking sharp above water.

While this battleship will cost a hefty $8 million, it will, in fact, be a permanent installment. This is another great aspect of the cofferdam: it can be both temporary or fixed. The permanent cofferdam enables future maintenance and repair work on structures like the U.S.S. North Carolina.

2. The Hoover Dam

It may seem counterintuitive to say that dams are made by using dams. But with this impressive dam that’s become an icon of the American road map, cofferdams were a huge part of the construction.

The Hoover Dam construction was an architectural and engineering feat in Nevada in 1933. Before the dams were installed, workers removed 250,000 cubic yards of silt from the river in order to ensure a solid starting foundation.

Two cofferdams were required to make sure the construction was dry and water-free. Both were made from earth and rockfill, and relied on an additional rock barrier to prevent any additional water seepage. While some people were worried that the spring Nevada floods may damage all of this foundation pre-work, the damming worked and construction went along as planned.

3. Ancient Roman Bridges

When we said that the cofferdam has been around for a really long time, we meant it. For thousands of years, civilizations have found the cofferdam useful, and you see this in many of the bridges of Ancient Rome.

Early populations relied on more basic forms of the cofferdam in order to control waters for drinking supply, irrigation, and land control. Often this entailed the diversion of a river. Legend has it that King Cyrus of Persia used the cofferdam in order to divert the Euphrates River in his pursuit of the city of Babylon. This meant that an entirely new empire was established based off of the use of this dam alone!

Similarly, the Romans made use of this handy type of damming when bridging the Danube River. Trajan’s Bridge, built as a result of cofferdam wood pilings, enabled the Romans to travel to contemporary Romania. This bridge totaled nearly 4500 feet in length.

4. The Tapan Zee Bridge, New York City

The Tapan Zee provides a great example of how cofferdams still help with important construction feats today. This incredible bridge spanning the Hudson River cost nearly $4 billion to construct. Its completion would not have been possible without the use of the cofferdam.

A complex software was used to design the steel dams, 90 feet by 45 feet, used in construction. The software also took soil type into consideration. Because the Hudson contains a lot of river silt and soft deposits, the Tapan Zee dams had to be backfilled in order to create a solid base for the bridge piers.

5. The La Belle ship

The La Belle shipwreck has long been an icon of the Texas coast, and the cofferdam made sure that La Belle remained a fixture of seventeenth-century history.

In 1687, this ship crashed along the shoreline as a result of poor weather and difficult seas. Manned by a New World explorer, this ship was the last of four ships sent to explore the unknown coasts. When La Belle crashed and sank, it became sealed in mud for over three hundred years.

In 1995, an archeological team discovered the site of La Belle’s sinking. Such a recovery requires a lot of complicated engineering. The Texas Historical Commission constructed a cofferdam system around the sunken ship. This elaborate system cost over $2 million.

The mission was successful, and in 1997 the full extent of the treasure was known. Hundreds of incredibly preserved artifacts and much of the ship’s original structure were recovered. If it weren’t for the cofferdam, we would never know this history.

Cofferdams of the Future

There’s no doubt about it: the cofferdam is versatile, useful, and amazing. It has enabled people to bring history back to the surface, cross rivers, and construct impressive architecture. The cofferdam will continue to be an essential part of contemporary engineering projects.

At DH Glabe & Associates, cofferdams are our bread and butter when it comes to earth shoring engineering. To date, we’ve completed over five thousand company projects in thirty-two years, relying on the expertise of over fifty professional licenses. We assist with both civil and commercial projects using a variety of technology, including H-piles, mechanically stabilized earth walls, sheet piles, geofabric, and secant pile walls.

Earth shoring is not all we do. No matter the size or type of your engineering project, at DH Glabe & Associates we pledge to be with you every step of the way. Contact any of our construction engineering experts today to learn about what we can do turn your project into a reality this year.

What You Need to Know: Earthquake Resistant Buildings

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Earthquake resistant buildings save lives. They limit property damage and comply with the latest seismic building codes.

If you do business in high earthquake hazard areas, here’s what you need to know about seismic building codes.

1. Seismic Building Codes are Getting Tougher

In 2015, Los Angeles overhauled their seismic regulations. 15,000 buildings needed retrofitting to better withstand the effects of earthquakes.

For decades, safety advocates worked to pass ordinances strengthening two types of structures. First were the brittle concrete buildings on L.A.’s major boulevards. Second, the boxy wood-frame apartment buildings built on top of carports. Over 65 people died when these types of buildings collapsed during earthquakes in 1971 and 1994.

2. Designing Earthquake Resistant Buildings is a Regional Endeavor

Building codes are based on the base shear formula. This formula measures how much earthquake-generated shear force will try to push the house off the foundation base. The simple formula multiplies the expected ground acceleration by the building’s weight.

But there’s no set amount for anticipated ground acceleration. For example, Los Angeles anticipates a different base shear than the California Building Code does.  The International Existing Code’s ground acceleration is different still. Keeping this in mind, it’s always best to use a base shear that’s tailored to your geographic region.

3. It’s Not Just the Building, It’s The Ground Underneath

Earthquake resistant buildings are great. But let’s say a building’s foundation sits on soft soil. Despite the advanced engineering techniques used, it could still collapse in an earthquake.

But, if the soil beneath a structure is solid, engineers can improve how the entire building foundation system responds to seismic activity.

One example is base isolation. In this method, a building is floated above the foundation on bearings, springs or padded cylinders. A solid lead core is used for vertical strength with rubber and steel bands for horizontal flexibility. This allows the foundation to move without moving the structure above.

4. Seismic Engineering has a Bright Future

All around the world are examples of newer structures withstanding earthquakes. One example is the Transamerica Pyramid in San Francisco.

During the Loma Prieta quake, the building shook for more than a minute and the top floor swayed a foot side to side. A deep concrete and steel foundation and a buttressed exterior allowed the building to escape structural damage.

Sensor readings were taken from the building’s frame and processed by the U.S. Geological Survey. The results showed the building could withstand an even larger seismic event.

The future of seismic engineering doesn’t just look forward. Retrofitting older buildings is as important as new construction. One bright spot is engineers are effectively and economically adding base-isolation systems to existing structures.

After the 1989 Loma Prieta quake, engineers retrofitted the city halls of San Francisco, Oakland, and Los Angeles. These earthquake-resistant structures will be tested. When and how remains to be seen.

The Final Analysis

More jurisdictions are mandating seismic building code compliance. That’s where DHG comes in. Our experience with earthquake resistant design ensures your clients’ buildings will comply with the latest codes.

To see our seismic engineering services up close, contact us for a consultation.

Stresses of Thermal Loads

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A thermal load is defined as the temperature that causes the effect on buildings and structures, such as outdoor air temperature, solar radiation, underground temperature, indoor air temperature and the heat source equipment inside the building.

ASCE 7-15 section 2.3.5 and 2.4.4 specifically mention thermal and other self-straining loads are to be considered, where applicable. For many cases, thermal movements cannot be restrained and instead designs need to allow for the structure/equipment to move thermally otherwise stresses in either the restraints or in the structure/equipment may cause catastrophic failures.

Different materials have different expansion rates. Structures or items with different types of materials connected by fasteners or adhesives can warp and break at extreme temperatures. For example, PE pipe will expand/contract around ten times more than steel pipe.

Historically, thermal stresses have caused failures in railroad tracks, roads and building facades and even electronic devices. Understanding these effects, and how to minimize them, reduces the risk of damage or failure at extreme temperatures and prevents having to perform costly repairs.

For example, a 200 ft long PE pipe can change in length by 1/8 of an inch per degree (F) of temperature change. If this movement is restrained, stresses in the pipe and the restraints will be generated. Depending on the strength of the restraint and the buckling strength of the pipe, the restraint could fracture or the pipe could buckle. Buckling pipes can injure anyone working next to the pipe and could also cause leaking of the pipe. Damage and injury could also occur when a restraint breaks.

Even sidewalks are not immune from thermal stresses. Recently a large section of 4’ wide sidewalk was installed about a block long. Thermal expansion joints were not provided at adequate spacing. During a hot summer day, a loud explosion was heard throughout the neighborhood. The sidewalk had buckled and one of the sections of sidewalk had shattered. The concrete was left uneven and damaged requiring removal and repair of the damaged area of the sidewalk. Additional thermal expansion joints were provided to hopefully prevent future problems.

DHG Hiring A Structural Engineer

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DH Glabe & Associates is looking for a well-qualified structural engineer who is interested in becoming the newest member of our team. This position will be headquartered at our corporate office in Westminster, CO. We are specifically seeking a candidate who appreciates variety in their work as we provide services for numerous types of projects and niches within the structural engineering realm.

We are currently seeking an individual to fill the position of Structural Design Engineer.  The responsibilities of this person will include but not be limited to design, engineering, drafting, client relations, project inspections, and site meetings. This person will be responsible for all tasks involved in taking a project from inception to completion in an autonomous environment.  This is primarily a design engineering position where the work consists of 45% engineering & calculations, 45% drafting and 10% field/site visits. DHG offers a competitive compensation package for the successful candidate. DH Glabe & Associates is an Equal Opportunity Employer.

Read more about this position and apply HERE.

Can Scaffolds Support This?

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Spring is in the air, the birds are chirping and scaffolds are being built. Can life get any better? It used to be that contractors feared winter in the northern regions of North America. Cold temperatures, snow, wind and generally miserable conditions prompted owners and contractors to curtail outdoor activities. That was then; now construction charges ahead, fearless and courageous against even the nastiest of weather. Once again science and progress has prevailed! Improved, clothing, materials, equipment and methods allow construction to continue in any environment.

 To facilitate these activities, it was common to enclose supported scaffolds against the weather. But times have changed; cold weather isn’t the only reason to enclose scaffolds. Containment of debris, tools and workers are now common reasons to enclose scaffolds. Enclosures are also used to advertise, block the work activities from pedestrians and even hide the workers who might be gawking at the pedestrians. Enclosing supported scaffolds is now a year around activity in all areas of North America, on all types of projects in all types of conditions.

Unfortunately, workers have false perceptions concerning supported scaffolds that are enclosed, including the perception that the forces on enclosed scaffolds are not as severe in summer as they are in winter; the perception that using open netting results in lower forces than using solid material; that no additional measures must be taken when a scaffold is enclosed and; site conditions have little effect on an enclosed scaffold.

The truth of the matter is that all scaffolds must be designed by a qualified person, that is, someone who can demonstrate the ability to properly design a scaffold, whether it is enclosed or not. Since designing for wind forces is a necessarily complicated matter, it is common that the qualified person for this design work is a Professional Engineer qualified in such activities. Of course, anyone can take a shot at the design (and unfortunately it is often the case), but the results can be fatal due to a gross underestimation of the forces developed by the wind. So, what is so complicated about wind design? Here are a few factors that must be considered:

Wind Forces

It is absolutely true that the force applied to a scaffold and its enclosure from the wind can be calculated. Short of a meteor falling out of the sky, there is no such thing as a “freak act of nature.” Those who argue so because their scaffold fell over need to be retrained. More accurately, an enclosed scaffold can be designed for a certain maximum wind speed; if the wind is expected to be higher than the design speed, either the scaffold must be dismantled, the enclosure removed, or additional measures must be taken to ensure the stability of the scaffold.

Wind Speed

Obviously, the wind velocity (speed) is the main factor in determining wind forces on a scaffold. However, choosing the correct wind speed for a specific location isn’t that easy. Although wind charts have been developed for North America that indicate maximum design wind velocities, choosing the correct velocity is just the starting point. In fact, there are numerous areas of the continent that have “special wind regions” that require additional investigation to determine the expected wind velocity. One example is along the east side of the Rocky Mountain range, extending from Montana down through Colorado and into New Mexico. At certain times of the year, Chinook winds, that is winds that drop down the east slopes of the mountains, reach as high as 100 mph. Similar winds, called the Santa Ana winds, occur in southern California. These winds don’t occur throughout the year; if your enclosed scaffold is erected during the right time of the year you don’t have to design for these winds; but watch out if the job is delayed and the scaffold is still standing when a Chinook wind hits!

Stability Ties

The key to scaffold success is to adequately design the scaffold and its connection to the adjacent structure. While U.S. federal OSHA and other agencies specify the minimum tie requirements for supported scaffolds, the tie spacing most likely will be grossly inadequate for any substantial enclosed scaffold. While #9 or #12 wire may suffice for a connection of an unenclosed scaffold, it typically is never adequate for an enclosed one. In other words, the ties for an enclosed scaffold must be designed for the anticipated tension and compression loads that are expected to occur. For those who choose to wing it and do something such as doubling up the ties should expect to see their scaffold take wing and fly like a kite. Keep in mind that it is not uncommon to have ties (and the adjacent structure) designed to hold several thousand pounds or more.

Adjustment Factors

When a qualified person designs an enclosed scaffold, he or she must consider these factors:

  • The height of the scaffold
  • The geographical location of the scaffold
  • The location of the scaffold relative to the surrounding structures
  • Surrounding Structures
  • Shape of the Scaffold/Structure (e.g. round or square)
  • Local Wind History
  • Partial or Full Enclosure
  • New construction or demolition
  • Existing structures—are the windows open or closed?

Time of year

This is not a complete list but it gives an idea of the potential complexity of the analysis and design.

Enclosure Porosity

Porosity is the fancy word for how many and how big are the holes in your enclosure material. If you are using netting, the holes can be quite small or they can be big. If the holes are over 2 inches in diameter, such as plastic fencing, porosity can be considered. Otherwise, the prudent scaffold designer will consider the netting as a solid material for the simple reason that the holes can become plugged. Snow and ice can easily plug the most porous netting in winter while sawdust, sand, asbestos (why you would use netting to try to contain asbestos is the more important question – you really need retraining!), stucco, plaster and other fine materials will also have an adverse effect on the airiness of your material regardless of the time of year.

While this article doesn’t cover all the factors that must be considered by the qualified person when designing an enclosed scaffold, it offers a glimpse into the complexity of the situation. Merely “doubling up the ties” and “this is the way I have always done it” is not a prudent approach; it just shows you are lucky. And while being lucky may work in craps or roulette, it has no place in the design of an enclosed supported scaffold. Is your life worth a throw of the dice?

Concrete Formwork – Under Pressure!

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Anyone who has worked in the construction industry is likely all too familiar with the term or feeling – “Under Pressure.” In this article I am focusing on concrete formwork engineering pressure, that is.  This article is not going to provide you with a ten step list to have a more peaceful construction related career. I will give you one generic management bite, and that is, “in construction do not let the urgent less important stuff squeeze out the important but less urgent stuff.” Now that is over, I can write about real physical pressure.

For engineering and physics purposes, pressure is simply a mass/weight divided by an area. Fluid pressure builds with the height of fluid. If a submarine goes too deep in the ocean, the fluid pressure can eventually crush the hull. Fluid pressure is calculated by simply multiplying the densify of the fluid by the depth (or height of fluid above). Back to the submarine, the pressure at 10 feet below water is 624 psf, at 1000 ft, you are at a pressure of 62,400 psf. This is also why the 10 ft long snorkel has not taken off in the vacation resorts, your lungs are not strong enough to suck in air past a depth of a couple feet.

Water is easy to visualize but not what I’d call a “common construction material”. However, soils and concrete are commonly designed with the same principle. Concrete has as a fluid has a density of about 150 lbs per cubit (larger than water), so a four foot tall wall form with wet concrete is going to have a pressure of around 600 psf at the bottom of the form. Soil has a typical fluid density of around 130 lbs per cubic foot, so a wall holding back four feet of wet fluid soil is typically modeled as having a pressure of around 520 psf at the bottom of the wall. So, all soil is not fluid like water, therefore geotechnical engineers typically account for this by providing an equivalent fluid pressure for the design. Typical values are between 30-60 lbs per cubic foot, although higher values can occur, especially, if expansive soils conditions exist. So, holding back 10 feet of soil is similar to holding back 8-10 feet of water. Holding back concrete with a height of 10 feet (all fluid) is similar to holding water with a height of 20 feet. This explains why so many forms blowout if not designed!

Remember, similar to you feeling the pressure of stacks of papers / task pile on your desk or mind, concrete, water, and soil increase in pressure with height or depth! A kiddy pool with only 1 ft of water is not a dangerous thing, but a concrete form with 4 ft of wet concrete has some significant pressure and think twice before you fill up that 20 ft tall column form or use plywood and 2x4s to hold back the slope!

Existing Structure Shoring

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Shoring existing structures can be a tricky business and the older the building, the trickier it can become.  Many older structures do not have drawings of the existing construction and if they do, they are not always reliable.  Many buildings go through generations of remodel with additions, renovations and improvisations that are not always documented properly.  Without proper documentation, it is sometimes difficult to determine the load bearing members in an existing building and this makes it difficult to shore.  If you can’t figure out where the loads are concentrated, you can’t figure out how to safely and economically support anything.

When undertaking the task of existing structure shoring you should consider consulting an engineer – and I don’t just say that because I happen to be an engineer!  The peace of mind that you get from entrusting this work to an engineer far outweighs the risk of liability if something goes wrong during the shoring operation. 

Things that your engineer will need to know before starting a shoring plan include the type of work being performed, the boundaries of work, distance to any excavation, dimensions of the building and location of load bearing members.  Other pertinent information includes the dead load of the supported area and any anticipated live loads – for example, will an office building remain occupied or is your customer trying to keep the parking garage operational during construction?  Depending on the scope of the job, snow and wind loads may also need to be taken into account.  Be certain to consider any special circumstances like required access openings in the shoring plan and work sequencing that would affect the standing shores.  Drawings, schematics and photographs can be provided to convey most of this information but, in some cases, it is easier and most cost effective for the person designing the shoring plan to visit the site.

If an existing structure is improperly shored, there is danger of damaging the building or of a collapse.  Providing as much accurate information as possible to your shoring designer will help to minimize risk and ensure the most accurate and economical design.  Don’t take chances, if in doubt get a professional engineer involved and maximize your chances of shoring success!